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Electronic structure of Sr2RuO4 studied by angle-resolved photoemission spectroscopy
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 310 (2007) 678–680 www.elsevier.com/locate/jmmm
Electronic structure of Sr2 RuO4 studied by angle-resolved photoemission spectroscopy H. Iwasawaa,b, Y. Aiurab,, T. Saitoha, Y. Yoshidab, I. Haseb, S.I. Ikedab, H. Bandob, M. Kubotac, K. Onoc a Department of Applied Physics, Tokyo University of Science, Shinjuku-ku, Tokyo 162-8601, Japan National Institute of Advanced Industrial Science and Technology, Tsukuba, Ibaraki 305-8568, Japan c Photon Factory, Institute of Materials Structure Science, Tsukuba, Ibaraki 305-0801, Japan
b
Available online 10 November 2006
Abstract Electronic structure of the monolayer strontium ruthenate Sr2 RuO4 was investigated by high-resolution angle-resolved photoemission spectroscopy. We present photon-energy (hn) dependence of the electronic structure near the Fermi level along the GM line. The hn dependence has shown a strong spectral weight modulation of the Ru 4dxy and 4dzx bands. r 2006 Elsevier B.V. All rights reserved. PACS: 74.70.Pq; 74.25.Jb; 79.60.-i Keywords: Electronic structure; ARPES
1. Introduction The monolayer strontium ruthenate, Sr2 RuO4 , has attracted huge interest in connection with the hightransition temperature (high-T c ) cuprates, because of its exotic properties such as the spin-triplet p-wave superconductivity with T c 1:5 K [1]. The ruthenate is the only known layered perovskite superconductor that is isostructural to the high-T c cuprates [2]. However, the electronic and magnetic properties of the ruthenate quite differ from those of the cuprates despite the structural similarity. In the ruthenate, Ru 4d t2g multibands, namely, Ru 4dxy;yz;zx –O 2p bands compose the electronic structure in the vicinity of the Fermi level (E F ), while in the cuprates the single band of Cu 3dx2 y2 –O 2p plays a crucial role. From a viewpoint of multibands vs. single band, therefore, a comparison of the ruthenate with the cuprates will shed light on the understanding of their unconventional p- or d-wave superconductivity. Corresponding author.
E-mail address: [email protected] (Y. Aiura). 0304-8853/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2006.10.178
Angle-resolved photoemission spectroscopy (ARPES) is an important probe of the electronic structure in a solid. In the ruthenate, however, earlier ARPES results [3,4] with respect to the Fermi surface (FS) were in some contradictions with the de Haas-van Alphen (dHvA) results [5] and the band-structure calculations [6–8]. This stems from the existence of an additional surface state near the M (p,0) point caused by a surface reconstruction [9–11]. Interestingly, recent ARPES studies have shown that the surface state near the M point was observed for the fresh surface [10,11], but was almost eliminated by aging the sample surface in situ [12]. In order to obtain fine details of bulk state, we performed photon-energy (hn) dependence of the electronic structure near the M point for an aged surface using high-resolution ARPES. The hn dependence has found a strong intensity modulation of the Ru 4dxy and 4dzx bands. 2. Experimental High-quality Sr2 RuO4 single crystals (T c 1:36 K) were grown by the floating zone method with a self-flux
ARTICLE IN PRESS H. Iwasawa et al. / Journal of Magnetism and Magnetic Materials 310 (2007) 678–680
technique [1,13]. The ARPES measurements were performed at the Photon Factory (KEK, Tsukuba) at undulator beamline (BL-28A), using a Scienta SES2002 electron spectrometer. The sample goniometer provides independent polar and tilt rotations of the sample (R-Dec Co. Ltd., i GONIO LT) [14]. The clean and flat surface of the samples was obtained by cleaving in situ in ultrahigh vacuum better than 2 10 10 Torr at 160 K to prevent the rotation of the RuO6 octahedron at the surface [11]. The replica of FSs in the bulk due to the surface rotation was not shown on the aged surface used here, and the coherent peak dispersion and line shape were consistent with the dHvA results and the band-structure calculations [5–8]. Present data were measured with several photon energies (35–65 eV) at 20 K along the GM line. 3. Results and discussions Fig. 1 shows the hn dependence of ARPES spectra along the GM line, taken with (a) 65 eV and (b) 42 eV photons, respectively. Top and bottom panels display the momentum distribution curve (MDC) at E F and ARPES intensity plot, respectively. At first sight, there are two bands crossing E F , which compose two electron-like FS sheets g and b, derived from the in-plane 4dxy and the out-of-plane 4dzx orbitals. Here, the Fermi momentum (kF ) is set to the momentum of g band crossing E F . The spectral weight of the g band is significantly suppressed compared with the b band at 65 eV photons. In the case of 42 eV photons, on the other hand, the g band has almost the same intensity as the b band. These observations are further confirmed from the MDCs at E F as shown in the top panels. With 42 eV photons, we performed the high counting statistics measurement as shown in Fig. 2 (a). Fig. 2 (b)
679
Fig. 2. (a) The high counting statistics ARPES intensity plot of Sr2 RuO4 at 42 eV photons along the GM line. (b) The electron dispersions obtained by fitting MDCs with Lorentzian curves.
shows the electron dispersions obtained by fitting MDCs with Lorentzian curves. Owing to the high counting statistics of the ARPES spectra and almost the equal intensity of the g and b bands, we were able to distinguish and extract each band dispersion up to a high-energy region beyond 100 meV. This is demonstrating that controlling the peak intensity modulation due to the photoemission matrix element effect plays an essential role to investigate fine details of the electron dispersion in a wide energy region in this material. Our precise results showed that the clear kink in the dispersion was observed for the g band around 40 meV, while not for the b band, consistent with our previous ARPES study [15]. Its origin is under investigation in detail with respect to the phonons and magnetic excitations. 4. Summary In summary, we have investigated the electronic structure of Sr2 RuO4 by high-resolution ARPES measurements. The hn dependence has found a strong intensity modulation of the two bands. In the ruthenate, it is essential to consider the effect due to the photoemission matrix element, in order to investigate fine details of the electron dispersion. Acknowledgments Authors thank the users’ working group for valuable help during the construction of the ARPES end station at the BL-28 of Photon Factory (KEK, Tsukuba). The sample goniometer was partly developed under a Joint Development Research at High Energy Accelerator Research Organization (KEK, Tsukuba). This work has been done under the approval of the Photon Factory Program Advisory Committee (Proposal No. 2006S2-001).
Fig. 1. ARPES intensity plot of Sr2 RuO4 along the GM line, taken with the photon energies (a) 65 eV and (b) 42 eV. Top and bottom panels display the momentum distribution curve at E F (open circle) together with the fitting curve (solid line) and ARPES intensity plot, respectively.
References [1] Y. Maeno, et al., Nature 372 (1994) 532. [2] Y. Maeno, et al., J. Low Temp. Phys. 105 (1996) 1577.
ARTICLE IN PRESS 680
H. Iwasawa et al. / Journal of Magnetism and Magnetic Materials 310 (2007) 678–680
[3] T. Yokoya, et al., Phys. Rev. Lett. 76 (1996) 3009; T. Yokoya, et al., Phys. Rev. B 54 (1996) 13311. [4] D.H. Lu, et al., Phys. Rev. Lett. 76 (1996) 4845. [5] A.P. Mackenzie, et al., Phys. Rev. Lett. 76 (1996) 3786. [6] T. Oguchi, Phys. Rev. B 51 (1995) 1385. [7] D.J. Singh, Phys. Rev. B 52 (1995) 1358. [8] I. Hase, Y. Nishihara, J. Phys. Soc. Japan 65 (1996) 3957.
[9] [10] [11] [12] [13] [14] [15]
A. Damascelli, et al., Phys. Rev. Lett. 85 (2000) 5194. K.M. Shen, et al., Phys. Rev. B 64 (2001) 180502. R. Matzdorf, et al., Science 289 (2000) 746. S.-C. Wang, et al., Phys. Rev. Lett. 92 (2004) 137002. S.I. Ikeda, et al., J. Crystal Growth. 787 (2002) 237. Y. Aiura, et al., Rev. Sci. Instr. 74 (2003) 3177. H. Iwasawa, et al., Phys. Rev. B 72 (2005) 104514.
Journal of Magnetism and Magnetic Materials 310 (2007) 678–680 www.elsevier.com/locate/jmmm
Electronic structure of Sr2 RuO4 studied by angle-resolved photoemission spectroscopy H. Iwasawaa,b, Y. Aiurab,, T. Saitoha, Y. Yoshidab, I. Haseb, S.I. Ikedab, H. Bandob, M. Kubotac, K. Onoc a Department of Applied Physics, Tokyo University of Science, Shinjuku-ku, Tokyo 162-8601, Japan National Institute of Advanced Industrial Science and Technology, Tsukuba, Ibaraki 305-8568, Japan c Photon Factory, Institute of Materials Structure Science, Tsukuba, Ibaraki 305-0801, Japan
b
Available online 10 November 2006
Abstract Electronic structure of the monolayer strontium ruthenate Sr2 RuO4 was investigated by high-resolution angle-resolved photoemission spectroscopy. We present photon-energy (hn) dependence of the electronic structure near the Fermi level along the GM line. The hn dependence has shown a strong spectral weight modulation of the Ru 4dxy and 4dzx bands. r 2006 Elsevier B.V. All rights reserved. PACS: 74.70.Pq; 74.25.Jb; 79.60.-i Keywords: Electronic structure; ARPES
1. Introduction The monolayer strontium ruthenate, Sr2 RuO4 , has attracted huge interest in connection with the hightransition temperature (high-T c ) cuprates, because of its exotic properties such as the spin-triplet p-wave superconductivity with T c 1:5 K [1]. The ruthenate is the only known layered perovskite superconductor that is isostructural to the high-T c cuprates [2]. However, the electronic and magnetic properties of the ruthenate quite differ from those of the cuprates despite the structural similarity. In the ruthenate, Ru 4d t2g multibands, namely, Ru 4dxy;yz;zx –O 2p bands compose the electronic structure in the vicinity of the Fermi level (E F ), while in the cuprates the single band of Cu 3dx2 y2 –O 2p plays a crucial role. From a viewpoint of multibands vs. single band, therefore, a comparison of the ruthenate with the cuprates will shed light on the understanding of their unconventional p- or d-wave superconductivity. Corresponding author.
E-mail address: [email protected] (Y. Aiura). 0304-8853/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2006.10.178
Angle-resolved photoemission spectroscopy (ARPES) is an important probe of the electronic structure in a solid. In the ruthenate, however, earlier ARPES results [3,4] with respect to the Fermi surface (FS) were in some contradictions with the de Haas-van Alphen (dHvA) results [5] and the band-structure calculations [6–8]. This stems from the existence of an additional surface state near the M (p,0) point caused by a surface reconstruction [9–11]. Interestingly, recent ARPES studies have shown that the surface state near the M point was observed for the fresh surface [10,11], but was almost eliminated by aging the sample surface in situ [12]. In order to obtain fine details of bulk state, we performed photon-energy (hn) dependence of the electronic structure near the M point for an aged surface using high-resolution ARPES. The hn dependence has found a strong intensity modulation of the Ru 4dxy and 4dzx bands. 2. Experimental High-quality Sr2 RuO4 single crystals (T c 1:36 K) were grown by the floating zone method with a self-flux
ARTICLE IN PRESS H. Iwasawa et al. / Journal of Magnetism and Magnetic Materials 310 (2007) 678–680
technique [1,13]. The ARPES measurements were performed at the Photon Factory (KEK, Tsukuba) at undulator beamline (BL-28A), using a Scienta SES2002 electron spectrometer. The sample goniometer provides independent polar and tilt rotations of the sample (R-Dec Co. Ltd., i GONIO LT) [14]. The clean and flat surface of the samples was obtained by cleaving in situ in ultrahigh vacuum better than 2 10 10 Torr at 160 K to prevent the rotation of the RuO6 octahedron at the surface [11]. The replica of FSs in the bulk due to the surface rotation was not shown on the aged surface used here, and the coherent peak dispersion and line shape were consistent with the dHvA results and the band-structure calculations [5–8]. Present data were measured with several photon energies (35–65 eV) at 20 K along the GM line. 3. Results and discussions Fig. 1 shows the hn dependence of ARPES spectra along the GM line, taken with (a) 65 eV and (b) 42 eV photons, respectively. Top and bottom panels display the momentum distribution curve (MDC) at E F and ARPES intensity plot, respectively. At first sight, there are two bands crossing E F , which compose two electron-like FS sheets g and b, derived from the in-plane 4dxy and the out-of-plane 4dzx orbitals. Here, the Fermi momentum (kF ) is set to the momentum of g band crossing E F . The spectral weight of the g band is significantly suppressed compared with the b band at 65 eV photons. In the case of 42 eV photons, on the other hand, the g band has almost the same intensity as the b band. These observations are further confirmed from the MDCs at E F as shown in the top panels. With 42 eV photons, we performed the high counting statistics measurement as shown in Fig. 2 (a). Fig. 2 (b)
679
Fig. 2. (a) The high counting statistics ARPES intensity plot of Sr2 RuO4 at 42 eV photons along the GM line. (b) The electron dispersions obtained by fitting MDCs with Lorentzian curves.
shows the electron dispersions obtained by fitting MDCs with Lorentzian curves. Owing to the high counting statistics of the ARPES spectra and almost the equal intensity of the g and b bands, we were able to distinguish and extract each band dispersion up to a high-energy region beyond 100 meV. This is demonstrating that controlling the peak intensity modulation due to the photoemission matrix element effect plays an essential role to investigate fine details of the electron dispersion in a wide energy region in this material. Our precise results showed that the clear kink in the dispersion was observed for the g band around 40 meV, while not for the b band, consistent with our previous ARPES study [15]. Its origin is under investigation in detail with respect to the phonons and magnetic excitations. 4. Summary In summary, we have investigated the electronic structure of Sr2 RuO4 by high-resolution ARPES measurements. The hn dependence has found a strong intensity modulation of the two bands. In the ruthenate, it is essential to consider the effect due to the photoemission matrix element, in order to investigate fine details of the electron dispersion. Acknowledgments Authors thank the users’ working group for valuable help during the construction of the ARPES end station at the BL-28 of Photon Factory (KEK, Tsukuba). The sample goniometer was partly developed under a Joint Development Research at High Energy Accelerator Research Organization (KEK, Tsukuba). This work has been done under the approval of the Photon Factory Program Advisory Committee (Proposal No. 2006S2-001).
Fig. 1. ARPES intensity plot of Sr2 RuO4 along the GM line, taken with the photon energies (a) 65 eV and (b) 42 eV. Top and bottom panels display the momentum distribution curve at E F (open circle) together with the fitting curve (solid line) and ARPES intensity plot, respectively.
References [1] Y. Maeno, et al., Nature 372 (1994) 532. [2] Y. Maeno, et al., J. Low Temp. Phys. 105 (1996) 1577.
ARTICLE IN PRESS 680
H. Iwasawa et al. / Journal of Magnetism and Magnetic Materials 310 (2007) 678–680
[3] T. Yokoya, et al., Phys. Rev. Lett. 76 (1996) 3009; T. Yokoya, et al., Phys. Rev. B 54 (1996) 13311. [4] D.H. Lu, et al., Phys. Rev. Lett. 76 (1996) 4845. [5] A.P. Mackenzie, et al., Phys. Rev. Lett. 76 (1996) 3786. [6] T. Oguchi, Phys. Rev. B 51 (1995) 1385. [7] D.J. Singh, Phys. Rev. B 52 (1995) 1358. [8] I. Hase, Y. Nishihara, J. Phys. Soc. Japan 65 (1996) 3957.
[9] [10] [11] [12] [13] [14] [15]
A. Damascelli, et al., Phys. Rev. Lett. 85 (2000) 5194. K.M. Shen, et al., Phys. Rev. B 64 (2001) 180502. R. Matzdorf, et al., Science 289 (2000) 746. S.-C. Wang, et al., Phys. Rev. Lett. 92 (2004) 137002. S.I. Ikeda, et al., J. Crystal Growth. 787 (2002) 237. Y. Aiura, et al., Rev. Sci. Instr. 74 (2003) 3177. H. Iwasawa, et al., Phys. Rev. B 72 (2005) 104514.